Not applicable.
Not applicable.
The invention relates to a device for the beam guiding of a laser beam with at least one optical element with a housing, which has a first housing section that is fittable at least partially into a support body of a power laser or beam guiding system or attachable thereto, and a further housing section that at least partially encloses the optical element.
Laser technology is used in a very wide variety of technical fields. For example, high-power lasers are used for flexible material processing. These power lasers have a resonator in which the laser light is generated. This resonator contains a back mirror as well as an output mirror, which is semitransparent so that a certain proportion of the laser light can leave the resonator in order, for example, to carry out the material processing. To increase the power in power lasers, it is necessary to increase the length of the discharge path in the resonator. In order to meet this requirement with a sufficiently small construction size, CO2 power lasers have been developed whose resonator is folded so that the light path is long despite a compact design. A separate gas discharge is generated in each path section of the folded CO2 power laser. Deflecting mirrors are provided at the end of the discharge path, so that the beam can be guided in the folded resonator.
The transmission and reflection factors of the optical elements used for the beam guiding are designed and matched differently according to the function. Optical elements that permit total reflection are employed, as well as optical elements that permit partial transmission with various transmission factors.
For deflecting a laser beam in a power laser, use is made of an optical element that routes the beam by total reflection. To that end, a device is provided which has a first housing section with a housing opening, which housing section is provided at least partially in a support body of the power laser. The opening of the first housing section is provided in the deflection region of the laser beam of the folded power laser. A second housing section is fixed to this first housing section by a screw connection, and is in turn mounted on the support body via a screw connection. This second housing section receives a holder that at least partially encloses the optical element. Using a further screw connection, this holder is mounted at the second housing section, so that the optical element is positioned at the opening in the first housing section and closes this opening.
The first housing section is made of stainless steel, in order to form a hard and even bearing surface at least in the edge region of the housing opening for the optical element. The second housing section is made of an aluminum alloy that is inexpensive to produce and permits good dissipation of the heat, which is absorbed during the deflection of the beam in the optical element. For better cooling, the holder of the optical element is designed in two parts and has, in a region opposite the housing opening of the first housing section, a cooling unit fitted into the holder with a feed channel and a discharge channel for the coolant. By passing the coolant close to the rear side of the optical element, better cooling is achieved in this region than in the second housing section and in the less thermally conductive first housing section.
During the operation of a power laser, the heating of the optical element and the heating of the two-part housing, consisting of different materials, leads to differential thermal expansion. Especially in the region where the first and second housing sections are screwed, increased stresses are induced which lead to unevenness of the bearing surface of the optical element and affect the deflection of the laser beam. The result of this is drifting of the mode when the stressed bearing surface is heated. This causes deviations in the intensity distribution of the mode, so that the achievable power and beam quality are reduced. At the same time, the unevenness of the bearing surface causes the optical element to tilt, which likewise has an effect on the mode drift.
It is therefore an object of the invention to provide a device for the beam guiding of a laser beam with an optical element, which permits an even and hard bearing surface for complete bearing of the optical element and good thermal dissipation.
This object is achieved by a device for beam guiding of a laser beam with at least one optical element and with a housing, which has a first housing section that is fittable at least partially into a support body of a power laser or beam guiding system or attachable thereto, and a further housing section that at least partially encloses the optical element, wherein a one-piece housing is provided, into which the optical element is fittable, and in that a bearing surface for the optical element is provided, which bearing surface at least partially encloses a housing opening and has a coating with a surface that is harder than a material from which the housing is made.
Through the combination of the one-piece housing with a harder surface than the housing material at the bearing surface for the optical element, an evenness is achieved which is increased by at least a factor of two compared with the prior art. The high degree of evenness that is achieved in this way has the advantage that improved thermal conduction is obtained by increased surface contact between the housing and the optical element, so as to dissipate the heat absorbed in the optical element. When the heating of the optical element and of the housing is reduced, the thermal effects that cause drifting of the mode are commensurately less.
Owing to the one-piece housing, it is possible to avoid stresses acting on the bearing surface due to different expansion coefficients of a material pair. The one-piece design also avoids stresses between the separate first and second housing sections due to the screw connection, so that a further interfering factor for the evenness of the bearing surface is eliminated. Furthermore, a cumulative effect of tolerances when assembling a first and second housing section can no longer occur.
Owing to the coating surface which is harder than the housing material, damage to the bearing surface when mounting the optical element, which leads to distortion of the bearing surface and would entail tilting of the optical element, can be prevented. Precise beam guiding is therefore achieved.
Furthermore, the configuration of the one-piece housing and of the coating provided on the bearing surface has the advantage of providing good thermal dissipation throughout the housing, especially from the region of maximum heating, the optical element, into the one-piece housing which extends to the immediate vicinity of the housing opening. At the same time, the one-piece housing has the advantage that uniform heating of the housing and of the holder for the optical element is obtained during operation, which leads to uniform expansion. The evenness of the bearing surface is therefore maintained even during operation.
Initial trials have shown that the configuration according to the invention can reduce the heating of the optical element by more than 20xc2x0 C.
Furthermore, the one-piece configuration of the housing can save on material costs as well as assembly costs.
The device has a housing made from a material with high thermal conductivity, for example a light metal alloy, in particular aluminum alloy. In this way, faster transport of heat outwards from the beam path and the optical element can take place, in order to keep the heating small. Aluminum alloys furthermore have the advantage that they can be processed easily and precisely.
According to another configuration of the invention, the coating of the bearing surface is provided using transition metals, for example nickel, molybdenum, chromium, rare earths or the like. This coating is preferably applied by a chemical process, vapor deposition process, by growth of layers or the like. Besides the said materials, it is also possible to provide other layers that have a hard surface and a high degree of evenness. Alternatively, a region enclosing the housing opening may also be treated by a hardening process, so that the bearing surface for the optical element has a hard surface with a high degree of evenness. In the case of coating processes that do not directly achieve the requisite evenness, the coating is post-processed.
The coating has a layer thickness of at least 20 xcexcm. This makes it possible to ensure that complete processing of the entire coating is possible after application, and a sufficient layer thickness is left. In fact, the preferred coating material nickel has a low thermal conductivity. Since the heat flux through the surface is proportional to the temperature difference and thermal conductivity, and inversely proportional to the layer thickness, the low thermal conductivity is compensated for by the extremely small layer thickness. Overall, for a fixed heat flux that needs to be dissipated, a very small temperature difference is obtained in the coating.
According to another configuration, the coated bearing surface is processed by turning or milling with diamond, polycrystalline diamond (PCD), ceramics as well as grinding, precision turning or lapping. An evenness of the bearing surface, which is less than 1 xcexcm, preferably less than 0.5 xcexcm, can therefore be achieved. Owing to the high-precision even bearing surface, good thermal conduction from the optical element into the housing is obtained. Furthermore, the optical element can be arranged in the device while being virtually free from any tilting.
According to another configuration of the invention, the housing has at least one cooling channel, which is provided in the housing level with the optical element and at least partially encloses the latter. The one-piece configuration of the housing makes it possible to provide the cooling immediately next to the optical element, in order to increase the thermal dissipation. Advantageously, a cooling channel is provided which is arranged symmetrically with respect to the optical element and fully encloses the optical element. In the ideal case, the cooling channel is designed in such a way that the shape of the cooling channel corresponds to the shape of the optical element; for example, in the case of a circular optical element, the cooling channel is annularly designed. Since an annular cooling channel is very expensive to produce in terms of manufacturing technology, the annulus is approximated, for example, by an equilateral parallelogram. A feed channel and a discharge channel open into the cooling channel, preferably while being offset from one another by an angle of 180xc2x0.
According to another configuration of the invention, the optical element is designed as a silicon or copper mirror, or made of zinc selenide, gallium arsenide or diamond, which may be used as a deflecting mirror, partial-transmission mirror, output element or back mirror, depending on the transmission and the reflection factor.
Owing to the low heating of the optical element by virtue of the inventive device for receiving the element, a silicon mirror for example, which has to date been used only in a power laser with a maximum power of 3 kW, may now even be used, for example, in a 4 kW power laser. These silicon mirrors are less expensive to manufacture than other optical elements that can be used with an increased laser power, and are therefore preferred.